Flip-chip technology presents a potentially highly effective way of fabricating a packaged semiconductor. The flip-chip mounting technique eliminates the use of bond wires between a chip or chip package and the substrate, resulting in increased reliability of the chip-to-substrate bond. In a flip-chip design, solder bumps are fabricated directly onto the aluminum bonding pads of the microchip. In this configuration, the active face of the chip is mounted face down, or “flipped” on the substrate. These bumps are then bonded directly to the package, or substrate pads, by reflowing the solder bumps. All bumps are bonded at the same time.
Designing microchips with flip-chip technology has numerous advantages. These advantages include a reduction in interconnection lengths, a smaller package footprint, and a lower package profile compared with conventional wire bonding techniques. In addition, flip-chip bonding allows for bonding locations within the interior of the chip, instead of just at the chip perimeter. Consequently, chips that are made with a flip-chip design have more I/O capacity than those chips of an identical size that are made with a perimeter interconnect design. Also, the very short lengths of the chip-to-package interconnect paths minimizes their inductance.
The solder bumps are formed on the microchip while the microchip is still in wafer form. A wide range of electrically conducting compositions are known for making the interconnection between flip-chip and substrate bond pads. Solder bumps, gold bumps, gold stud bumps, and other conventional metal bump configurations are known to the art. Both lead based and lead free solders are used for solder bumps. Desirable solders melt at the relatively high temperature of 315 degrees Celsius, which permits other low-melting-point solders to be used at in subsequent module-to-card, or card-to-board packaging level processes without reflowing the flip-chip bonds. In addition, the art has developed electrically conducting polymer compositions for flip-chip interconnection bumps. In a flip-chip fabrication process using polymer materials, electrically conductive polymer bumps are formed on the bond pads, typically of the flip-chip, and are polymerized or dried during bonding to the substrate bond pads. This fabrication process forms both an electrical and a mechanical adhesive bond between the flip-chip and the substrate bond pads.
Conventionally, once a flip-chip is bonded to a substrate, whether by metallic or by polymer bump interconnections between the chip and substrate bond pads, an underfill material is dispensed between the chip and the substrate. The underfill material is typically provided as a liquid adhesive resin that can be dried or polymerized. The underfill material provides enhanced mechanical adhesion and mechanical and thermal stability between the flip-chip and the substrate, and inhibits environmental attack of chip and substrate surfaces.
Due to its numerous advantages, it is highly desirable to utilize flip-chip technology in connection with Radio Frequency (RF) circuits and systems. However, at the present time, flip-chip technology that has high quality connections with RF circuits with plated copper inductors and transformers is unknown to the art. For microchips with integrated inductors and transformers, an underlying layer of metal is used for redistribution in the thin metallization system. This redistribution results in excessive losses for RF applications, thereby inhibiting the use of flip-chip technology for RF designs. It is therefore desirable to develop a flip-chip process and design that can provide high quality integration of inductors and transformers.